Hollow articles such as industrial shipping containers for foods, paints, construction material and so forth are often injection molded in large presses having axially aligned sections or portions which are adapt for axial movement relative to one another and which open and close cyclically during the molding operations. The mold tooling which is mechanically carried by the press typically includes a male mold element having a core portion which defines the interior configuration of the article and a female mold element having a cavity which defines the exterior configuration of the molded article. The male and female mold elements are telescopically interfitting along the forementioned axis of press travel.
The male and female mold elements are typically machined or configured to provide cambered or tapering striking surfaces which are outboard of the surfaces forming the mold cavity and which are brought into mutual contact as the mold elements are closed upon one another. The proper alignment and fit of these mold elements and particularly the striking surfaces is of utmost importance in the success of the molding operation in that such fit determines the quantity of flash or overflow from the mold cavity and also the uniformity of the wall thickness of the article about the longitudinal axis.
Excess flash is, of course, to be avoided as it represents wasteful usage of plastic material and requires trimming. However it is not nearly so important in the quality of the molded article as is the maintenance of a uniform wall thickness around the longitudinal axis of the article. If the mold elements, as a result of usage, warpage, thermal expansion or improper installation or maintenance, shift laterally from one another, the result is a progressive wearing of the striking surfaces and the ultimate seating of the two mold elements within one another in an axially misaligned condition such that the radial thickness of the mold cavity is greater on one side than the other. When filled, this results in an article in which the wall thickness on one side exceeds nominal or ideal thickness and the wall thickness on the diametrically opposite side is less than nominal or ideal thickness. The reduced wall thickness clearly reduces the structural strength of the container and may lead to rejection of the container during the inspection phase or a failure of the container in the field; both eventualities are undesirable.